1. Field of the Invention
The present invention relates to a photographic film cassette. In particular, the present invention is an improved photographic film cassette in which rotation of a spool causes a film leader to advance to the outside of a cassette shell.
2. Description of the Related Art
One conventional type of film cassette has 35 mm photographic film positioned so that the film leader does not protrude from the cassette shell prior to loading the film cassette into a camera. Such a film cassette is easily loaded. Simple feeding mechanisms are used with this type of film cassette including a construction which rotates the spool to unwind the photographic film thereby causing the film leader to move through a film passage mouth and exit the film cassette. Such a device is disclosed in U.S. Pat. Nos. 4,834,306 and 4,832,275.
In the above mentioned conventional film cassette, a spool is rotatably contained in a cassette shell which consists of upper and lower shell halves. A pair of annular ridges are formed on the inside surface of the cassette shell so as to contact the outermost turn of the roll of film to prevent it from loosening. A separating claw is provided inside the cassette shell for separating a film leader of the photographic film from the roll of film so as to direct it to a film passage mouth for a film initial advance. Therefore, the film leader is advanced to exit the film cassette through the film passage mouth by rotating the spool in the direction of unwinding the photographic film. It is noted that a pair of rings rotatably mounted on the inside cylindrical surface of the cassette shell, or a pair of circumferential ridges formed on the peripheral edges of the flanges, projecting toward the respective opposite flanges, may be provided, in place of the annular ridges, to prevent the roll of film from loosening.
FIG. 24 illustrates spool 7, having a construction in which photographic film 6 does not become detached from spool 7, even by fully drawing photographic film 6 from the cassette shell, as film trailer 5 is reliably fixed to spool 7. Slit 16 is formed in spool core 7a between flanges 9 and 10 for inserting film trailer 5 therein. A pair of anchoring claws 17 and pressing projections 18 are formed in slit 16. When film trailer 5 is inserted in slit 16, anchoring claws 17 are engaged with a pair of holes 15 formed in film trailer 5, so that film trailer 5 becomes anchored in slit 16, by virtue of engagement between holes 15 and anchoring claws 17, and held into engagement by contact with pressing projections 18, as illustrated in FIG. 27.
When spool 7 is rotated in the direction of winding up of photographic film 6 with trailer holes 15 anchored on claws 17, photographic film 6 is wound onto spool 7 with film trailer 5 bent on an edge of the entrance 16a of slit 16 at an angle of 90 degrees or less in the direction opposite to winding up of photographic film 6. A portion of photographic film 6 bent at the entrance to slit 16a is herein called film root portion 5b. Because film root portion 5b is so sharply bent on spool core 7a, the innermost turn of photograph film 6, close to film root portion 5b, is subjected to a force tending to restore the initial straight form of photographic film 6, due to its rigidity.
The portion of the innermost turn close to film root portion 5b thus loses contact with spool core 7a so that photographic film 6 is wound in a roll having an oval shaped, or eccentric, cross section with respect to spool core 7a. When spool 7 is rotated so as to unwind photographic film 6 under this condition, annular ridges formed on the inside of the cassette shell are opposed by an excessive force at the outermost position corresponding to film root portion 5b. This increases the torque necessary to cause film leader 5 to exit the cassette shell. The battery in the camera loaded with such a conventional film cassette is thus discharged much faster than is required because a relatively large load is applied to the motor in the camera when transporting photographic film 6.
Spool 7, as illustrated in FIG. 24, is also disadvantageous because it does not reliably transmit the rotation of spool 7 to the roll of photographic film 6 when spool 7 is rotated. This is so because when the film leader is actually propelled through the film passage mouth by rotating spool 7, the plush material, disposed inside the film passage for the purpose of limiting light entering therethrough, applies a load to the film leader because the film leader is forced to advance between upper and lower plush material pieces. This load is transmitted to the anchoring construction of film trailer 5 on spool 7 via the roll of photographic film 6.
Even though spool 7 is rotated in the direction so as to unwind photographic film 6, as indicated by arrow F in FIG. 26, the additional load on film leader 6a causes film trailer 5 to move from the anchoring position on spool 7 toward the exit of slit 16. Shoulder portions 5a, defined on the lateral sides of film trailer 5, by reducing the width of the photographic film 6, are abutted on spool core 7a as illustrated in FIGS. 26 and 27. Further rotation of spool 7 in the direction of F causes film trailer 5 to flex into a U-shaped cross-section reverse to the direction of convolutions of the roll of film 6. Shoulder portions 5a are thus subjected to a large pressure by contact with the entrance of slit 16. Accordingly, crack S, indicated by the broken line in FIG. 27, tends to occur in shoulder portions 5a. Accordingly, it is possible for film trailer 5 to be cut off when the applied pressure is high so as cause the film cassette to fail to advance film 6.